1. Field of the Invention
The present invention relates generally to a gradient-index rod lens having a core/clad structure as well as a rod lens array and an image processor that each includes the rod lens.
2. Related Background Art
A gradient-index rod lens (hereinafter also referred to simply as a “rod lens”) is a rod-shaped optical element having a refractive index distribution from its center to its periphery in the section thereof. The rod lens has many characteristics including the following, for instance: it can exhibit lens functions such as, for instance, an imaging function even when both the end faces thereof are flat; and lenses with microdiameters can be produced easily. Using such characteristics, a rod lens array in which the above-mentioned rod lenses are arrayed is employed for a wide range of uses as an imaging lens of an optical system of an image processor (for example, an image-forming apparatus such as an LED array printer, a liquid crystal shutter printer, etc, or an image scanner such as a facsimile, a scanner, etc.). Such a rod lens usually is formed by providing a mother glass rod with a refractive index distribution. A general method of providing the refractive index distribution is one that is carried out through ion exchange.
The mother glass rod is produced by a direct drawing method (a continuous drawing method) that has been used widely due to its excellent productivity. In the direct drawing method, for instance, as shown in FIG. 14, molten glass 101 that has been subjected to a degassing process (a fining process) as required is put into a crucible 103 that is kept warm with a heater 102. The molten glass 101 flows out of the lower end of a nozzle 104 located in a lower end portion of the crucible 103 while being cooled gradually inside the nozzle 104, and thereby is subjected to hot drawing. Thus a mother glass rod (a fiber) 105 with a diameter of about 0.1 mm to 4 mm can be formed continuously. The direct drawing method, however, has a disadvantage in that the molten glass 101 tends to devitrify when being cooled gradually inside the nozzle 104. Particularly, a glass composition that contains Li2O as a component to be employed for providing a refractive index distribution and that is substantially free from PbO tends to devitrify when being drawn by the direct drawing method.
In order to prevent such devitrification from occurring during the drawing, for instance, JP8(1996)-13691B2 discloses a method in which additives such as BPO4, Al2(PO3)3, etc. are added to molten glass.
Furthermore, JP5(1993)-72337B2 discloses a method of forming a mother glass rod 116 having a structure (a core/clad structure) composed of a core glass mother composition 111 and a clad glass mother composition 112 covering the core glass mother composition 111 by the direct drawing method (a concentric crucible drawing method) using a concentric crucible 115 including an inner crucible 113 and an outer crucible 114 (see FIG. 15). As shown in FIG. 15, the inner crucible 113 and the outer crucible 114 are charged with the core glass mother composition 111 and the clad glass mother composition 112, respectively, with the respective mother compositions being in a molten state. The respective mother compositions are allowed to flow out from a nozzle 117 located in the lower end portion of the concentric crucible 115 to form the mother glass rod 116. The mother glass rod 116 thus formed can be drawn out continuously with rollers 118. In this case, when a glass composition that tends not to devitrify is employed as the clad glass mother composition 112, the core glass mother composition 111 can be prevented from coming into contact with the nozzle 117 even in the temperature region where devitrification tends to occur in the case of using the direct drawing method. Accordingly, the core glass mother composition 111 can be prevented from devitrifying and thus the whole mother glass rod 116 can be prevented from devitrifying during the drawing. JP2004-151682A discloses an example of the clad glass mother composition that tends not to devitrify.
The mother glass rod having a core/clad structure (and a rod lens having a core/clad structure formed of this mother glass rod) can provide not only the effect of preventing devitrification from occurring during the drawing but also various effects through the selection of a suitable composition thereof. For instance, when a mother glass rod is formed that includes a clad glass mother composition to which a colorant is added, the rod lens formed thereof can reduce stray light to be caused therein (which is disclosed in, for instance, WO00/04409, JP10(1998)-139468A, JP10(1998)-139472A, and JP2001-255406A).
Furthermore, for instance, JP63(1988)-301901A discloses the following. A gradient-index element is formed in which an optical absorption part containing exchangeable ions and a colorant is present in at least a part of the surface of a transparent dielectric part having a refractive index distribution. The thermal expansion coefficient of the optical absorption part that is obtained after ion exchange is allowed to be equal to or lower than that of the vicinity of the interface between the transparent dielectric part and the optical absorption part. Thus the optical characteristics and strength of the element can be improved.
A mother glass rod having a core/clad structure often warps during ion exchange. Particularly, when a mother glass rod containing Li2O as a component to be employed for providing a refractive index distribution is subjected to ion exchange, warping may occur more frequently. In a rod lens array composed of warped rod lenses, it is difficult to secure its optical performance (i.e. the resolvable spatial frequency may be lowered or distortion may occur in an image that is transmitted). Hence, there is a demand for rod lenses that can be prevented from warping during ion exchange.
JP63(1988)-301901A discloses the technique of improving the strength of the element that has been subjected to ion exchange but includes no description about the technique of preventing the mother glass rod (a rod lens) from warping during ion exchange.